The Highest Oxidation State of Rhodium: Rhodium(VII) in [RhO3]+

Abstract Although the highest possible oxidation states of all transition elements are rare, they are not only of fundamental interest but also relevant as potentially strong oxidizing agents. In general, the highest oxidation states are found in the electron‐rich late transition elements of groups 7–9 of the periodic table. Rhodium is the first element of the 4d transition metal series for which the highest known oxidation state does not equal its group number of 9, but reaches only a significantly lower value of +6 in exceptional cases. Higher oxidation states of rhodium have remained elusive so far. In a combined mass spectrometry, X‐ray absorption spectroscopy, and quantum‐chemical study of gas‐phaseRhOn+ (n=1–4), we identify RhO3+ as the 1A1' trioxidorhodium(VII) cation, the first chemical species to contain rhodium in the +7 oxidation state, which is the third‐highest oxidation state experimentally verified among all elements in the periodic table.


Experimental details
[RhO n ] + (n = 0 − 3) molecular ions were produced by argon sputtering of a rhodium target in the presence of oxygen, which was introduced as a mixture of 1% oxygen in helium carrier gas. In the plasma region of the ion source, ozone is formed [1] that reacts to produce cationic, anionic and neutral rhodium-oxygen species of different sizes and compositions. [2][3][4] The magnetron sputter source was kept at room temperature.
The cationic species are directed via electrostatic fields and a radio frequency ion guide to a radio frequency quadrupole mass filter, where the ions of interest are selected. The mass selected ions are guided into a linear radio frequency quadrupole ion trap, which is cooled by liquid helium. The ion trap has a pulsed exit aperture where the stored ions are extracted in bunches, and are thereafter mass analyzed by a reflectron time-of-flight mass spectrometer. Fig. S1 shows the mass spectra of the investigatred species, [RhO n ] + (n = 0 − 3).
The ion trap is aligned with the beamline, allowing the interaction of the X-rays with the stored ions. X-ray absorption by the ions is followed by multiple Auger decay leading to the dissociation of the ions due to Coulomb repulsion. The ion yield spectrum is obtained by monitoring the product ion intensity with the time-of-flight mass spectrometer, while scanning the photon energy over an absorption edge. The incident photon energy was scanned in steps of 160 meV with a photon energy bandwidth of 345 meV at the oxygen K-edge and steps of 150 meV with a photon energy bandwidth of 300 meV at the rhodium M 3 -edge.
The oxygen K-edge spectra obtained from different ion yield channels are shown in Fig. S2. The most intense product ions, Rh 2+ and O + for samples [RhO 1,2 ] + , and Rh + and O + for [ RhO 3,4 ] + , resulted in very similar ion yield spectra for a respective parent ion, indicating that the partial ion yield spectra are a good approximation of the total ion yield, and thus to the X-ray absorption spectrum. The figure in the main text uses the ion yield channel that showed the highest signal-to-noise ratio, which is the O + yield in all cases.
For the rhodium M 3 -edge plots, only the Rh 2+ product ion was observed with significant intensity, therefore, all plots shown are from the Rh 2+ yield channel. All ion yield spectra showed in this work are normalized to a 0-1 intensity range.  2 Rhodium M 3 -edge spectra Fig. S3 shows the rhodium M 3 -edge spectra of the cationic rhodium-oxo series [RhO n ] + (n = 0 − 3), indicating the chemical shift on the position of the peak median along the series as the oxidation state (OS) of the rhodium atom increases.
In order to minimize the experimental uncertainty, only the energy range that comprises the absorption peak was considered to calculate the median (Table S1). We plot the integral value for this region, done with the raw data, as a function of the energy. We then extract the energy position of the point at half of the height of the integral curve. This process was done individually for two scans of each sample and the average value of the median was considered.   In this study, also the [RhO 4 ] + species was observed, although at much lower intensity, as evidenced by the smaller signal-to-noise ratio when comparing the mass spectrum of the selected [RhO 4 ] + (Fig. S4) with the other rhodium-oxo species in Fig. S1. The presence of a σ * -like transition around 540 eV at the oxygen K-edge of [RhO 4 ] + in Fig. 3 suggests the presemce of an O 2 unit. [5] The rhodium M 3 -edge spectrum is shown in Fig. S5. The peak median at the rhodium M 3 -edge was calculated subtracting a linear background from the raw data and extracting the half of the peak integral. The position of the Rh M 3 -edge median of [RhO 4 ] + is at 497.90 ± 0.15 eV, which when inserted in the equation of the linear fit showed in Fig. 3, predicts an OS = 4.6 ± 0.3 for the Rh atom in [RhO 4 ] + . Therefore, two possibilities arise, a cationic peroxo-superoxo system for OS(Rh) = 4, or a cationic diperoxo system for an OS(Rh) = 5, and a distinction could be made by a deeper theoretical study combined with these results. Though it is not clear how the oxygen atoms are bonded to the Rh atom in the [RhO 4 ] + system, it is evident that we do not have a cationic tetroxide system and the trioxide cation is the highest-oxidized rhodium-oxo system we could produce.
To unequivocally determine the electronic ground state, single point calculations at the optimized structures were performed at the CASSCF (14,20), CASSCF (15,20), NEVPT2, CASPT2 and CCSD(T) levels of theory. The CASPT2 calculations were carried out without IPEA shift. To account for scalar relativistic effects, the DKH Hamiltonian was used together with the aug-cc-pVTZ-DK [16] basis sets for the multi-reference methods CASSCF, NEVTP2, and CASPT2. The 14-orbital active space spans the MOs formed by the 4d(Rh), as well as the 2p(O) atomic orbitals. The 15-orbital active space additionally includes the MO mainly formed by the 5s(Rh) atomic orbital.
Geometry optimizations at the DFT and CCSD(T) levels of theory converge to trigonal planar structures (D 3h ) with bond lengths of 167 pm (B3LYP), 168 pm (M06L) and 169 pm (CCSD(T), see Table S5). The different electronic configurations arise by distributing the four lowest energy electrons in the a 2 , a 1 , and e frontier molecular orbitals (MOs) in the D 3h high symmetry reference geometry shown in Figure 1. In the 1 A 1 ground state, S 0 , the electrons are paired in the non-bonding a 1 and a 2 MOs, (a 2 ) 2 (a 1 ) 2 (e ) 0 . The vertical electron affinity calculated for the ground state at the CCSD(T) level is 10.6 eV (B3LYP: 11.1 eV) The lowest excited state, (a 2 ) 2 (a 1 ) 1 (e ) 1 (e ) 0 , is prone to Jahn-Teller distortion and geometry relaxation leads to two triplet states of lower symmetries, Tables S6 and S7. Their corresponding electronic configurations are (a 2 ) 2 (a 1 ) 1 (e θ ) 1 and (a 2 ) 2 (a 1 ) 1 (e ) 1 , respectively. The lowest 5 A 1 quintet state (Q 1 ) with an electron population of (a 2 ) 1 (a 1 ) 1 (e θ ) 1 (e ) 1 shows a highly symmetric D 3h point group symmetry. Lastly, the stationary point of 3 B 2 , the third triplet state (T 3 ) with an occupation pattern of (a 2 ) 2 (a 1 ) 0 (e θ ) 1 (e ) 1 exhibits C 2v symmetry, despite its totally symmetric charge distribution at the high symmetric reference geometry (Table S9). Vibrational analyses of the stationary points obtained at the DFT level were performed, see Tables S6-S10 that also lists the point groups in which the electronic structure calculations were performed.
The pre-edge region of the oxygen K-edge X-ray absorption spectrum of the S 0 , T 1 and T 2 states shown in Fig. S6 was calculated using TD-DFT following the procedure outlined by Ray [26]   The relative energies shown in Table S2 were calculated by using the lowest single point energy for the respective method at the given stationary point (Tables S3-S5). The singlet-triplet energy gap was predicted to be 17-75 kJ mol −1 . The almost identical energies gaps for both active spaces suggest that perturbative treatment of the CASSCF (14,20) recovers most of the correlation energy neglected by excluding the Rh(4s) orbital.
The CASPT2 method is suspected to systematically underestimate energies of openshell states. [27,28] A recent study has shown that this effect is minimal for organic chromophores using small basis sets, but often a stronger effect is still assumed for transition metal complexes in combination with larger basis sets. [29] Hence, the CCSD(T) and NEVPT2 energies of 46-64 kJ mol −1 can be considered a more reliable estimation of the singlet-triplet gap. Table S2: Energy levels of the lowest electronic states. The enthalpy differences (∆H, T = 0 K) were calculated from the electronic energies and the zero point energies.